In practice, a wireless communication transmitter emits energy in frequency regions other than those intended for the transmission. If not limited in some way, the unwanted emissions substantially interfere with other communication systems operating in these frequency regions. Wireless communication standards, therefore, impose limitations on a transmitter's out-of-band (OOB) emissions.
OOB emission requirements typically specify a minimum Adjacent Channel Leakage Ratio (ACLR) for each channel near the intended transmit channel. The ACLR is a ratio of the power emitted in the intended channel to the power leaked into a certain nearby channel.
In general, leakage into nearby channels directly relates to the operating point of the transmitter's power amplifier. The amount of power leaked into nearby channels, for example, increases when the power amplifier operates in its non-linear region, at higher power levels, due to intermodulation distortion. Accordingly, an effective method for achieving a pre-determined OOB emission requirement entails reducing or “backing off” the maximum transmit power on the intended transmit channel from its nominal value.
The amount by which the maximum transmit power must be backed off, while also accounting for amplifier efficiency, depends on the properties of the transmitted waveform (e.g., the modulation, spreading code, spreading factor, gain factors, etc). For some signals, these properties can be well quantified in terms of Cubic Metric (CM) or Peak-to-Average Power Ratio (PAPR), as described in e.g. 3GPP Technical specification TS25.101, Release 8. Such quantities, however, can be costly in terms of processing resources to compute quickly, making estimation of the required back-off upon a dynamic change in the properties of the transmitted waveform particularly problematic.
With various known approaches addressing this issue for single-carrier transmitters, PAPR or CM can be pre-computed for all possible configurations of the transmitted waveform and the corresponding required back-off stored in a look-up table. This approach, however, proves more and more impracticable as the number of configuration possibilities increases, due to the size of the required look-up table. A multi-carrier transmitter, for example, simultaneously transmits two or more separately modulated carriers, each of which occupies a distinct frequency region. See, e.g., multi-carrier operation outlined for inclusion in 3GPP Rel.9, “Dual-Cell HSUPA”, 3GPP Work Item Description, RP-090014. When the configuration on each carrier is independent from that of the other carriers, the number of possible configurations of the compound waveform (and thereby the size of the required look-up table) may be several orders of magnitude greater than in a single-carrier transmitter.